A wizard did it!

Awesome link. :D

I don't know why I like the guy's face at the last panel. I think it's the lack of nose.... yup. definitely it.

I think this needs to be known.
Last night, I had a dream with a duck in it, and it looked like a real life Question Duck. No joke.
Your comic has lasting psychological effects, and they're pretty awesome.

Because they're not really millions of years old.

Because they are in caves, so they are sheltered from wind, water, and sunlight. Also a wizard did it.

Because the ones that we find have generally been pretty sealed off from wind, moisture, and sunlight, which is why they don't let tourists come in an look at them anymore, because it quickly erodes away the paintings. Also the durability of the different cave paintings depend on what they were painted on and with. Also, the often think of cave paintings because we don't have wall paintings for a bit afterward in history, quite possibly because they were destroyed while the walls were broken or became eroded.